This invention relates to liquid ester compositions. More particularly, this invention relates to mixtures comprising a diester derived from a dialkylene glycol and benzoic acid or a substituted benzoic acid and a monoester derived from at least one dialkylene glycol or other dihydric alcohol and the same acid. The weight ratio of monoester to diester in the present compositions is sufficient to cause the compositions to be liquids at 28xc2x0 C. The diester is a solid at this temperature.
The liquid ester compositions of the present invention are particularly suitable plasticizers for aqueous polymer compositions useful as adhesives.
Mono- and diesters derived from benzoic acid and dialkylene glycols such as diethylene glycol and dipropylene glycol are known compounds. The use of these diesters alone and in combination with limited amounts of the corresponding monoesters as plasticizers for aqueous polymer emulsions is described in the prior art. As an example of the latter, U.S. Pat. No. 5,676,742, which issued to William Arendt on Oct. 14, 1997 describes plasticized aqueous polymer compositions useful as latex caulks. The primary plasticizer in these compositions is a diester derived from benzoic acid and diethylene and/or dipropylene glycol. Both hydroxyl groups of the diol are esterified.
In accordance with the teaching of the aforementioned patent to Arendt, a shortcoming of caulks formed from aqueous polymer compositions containing dibenzoates of diethylene- and/or dipropylene glycol as plasticizers is the greater susceptibility of the final caulk to fungal attack relative to caulks prepared using the same polymer composition and a diester of phthalic acid such as butylbenzyl phthalate as the plasticizer.
The Arendt patent teaches that even though monoesters of glycols are not considered effective plasticizers for many applications, relatively low levels of diethylene glycol (DEG) monobenzoate and/or dipropylene glycol (DPG) monobenzoate in combination with the corresponding dibenzoate as the primary plasticizer were sufficient to impart fungicidal properties to the caulks without any substantial adverse effect on the processability of the polymer compositions.
Throughout this patent the concentrations of the DEG and DPG monobenzoates in blends containing the corresponding dibenzoates are expressed in terms of a xe2x80x9chydroxyl numberxe2x80x9d. The units for the xe2x80x9chydroxyl numberxe2x80x9d are understood by those skilled in the art to be milligrams of potassium hydroxide per gram of sample.
The relationship between the xe2x80x9chydroxyl numberxe2x80x9d and the relative concentration of monobenzoate in a monobenzoate/dibenzoate blend will be explained in greater detail in a subsequent section of this specification.
The hydroxyl numbers exhibited by the mixtures of diethylene glycol mono- and dibenzoates disclosed in the Arendt patent range from 12.1 to 58.4. This is equivalent to a concentration of diethylene glycol monobenzoate in the mixture of from 4 to 18 weight percent.
It is understood by those skilled in the art that to be effective, a water-insoluble plasticizer should be absorbed by the particles of polymer present in an aqueous emulsion of the polymer. This absorption typically results in an increase in viscosity of the polymer composition. Data in Table 6 of the aforementioned Arendt patent demonstrate significantly less of an increase in viscosity with increasing plasticizer concentration as the hydroxyl number of the plasticizer is increased, which is equivalent to an increased concentration of the corresponding monobenzoate in a monobenzoate/dibenzoate blend. One skilled in the art of polymer chemistry would conclude from these data that the monobenzoates of diethylene and dipropylene glycols are not effective plasticizers for the aqueous polymer compositions described in the patent and would not employ these monobenzoates as plasticizers in aqueous polymer compositions unless fungicidal activity was a requirement for the final product.
The use of the monobenzoate of 2,2,4-trimethyl-1,4-pentanediol as a plasticizer for polyvinyl chloride is described in U.S. Pat. No. 5,006,585, which issued to DiBella on Apr. 9, 1991. Because polyvinyl chloride is processed as a molten material, any adverse effect of the plasticizer on the viscosity or processability of aqueous polymer dispersions is not a consideration. The only criterion for the melting point of the plasticizer is that it be below the melting point of polyvinyl chloride.
The present invention is based on the following unexpected findings.
1. The addition of at least about 30 weight percent of monobenzoates of a specified class of dihydric alcohols and glycols to diethylene glycol dibenzoate, which is a solid at 28xc2x0 C., results in a mixture that is a liquid at 12xc2x0 C. Liquid plasticizers are advantageous relative to solid ones because liquids are easier to transport and blend. They also eliminate the need to heat aqueous polymer compositions during or following addition of the plasticizer to ensure uniform distribution of plasticizer throughout the polymer composition and absorption of the plasticizer by the emulsified polymer particles.
2. The reduction in plasticizer efficacy with monobenzoate concentration demonstrated by the data in the aforementioned Arendt patent is not observed when the concentration of monobenzoate is increased beyond the limits taught in this patent. Preferred compositions exhibit an increase in viscosity at least equivalent to that achieved using ethylene gylcol dibenzoate.
U.S. Pat. No. 5,990,214, issued on Nov. 23, 1999 to Arendt et al. discloses mixtures comprising the dibenzoates of DEG and triethylene glycol. These blends exhibit a eutectic freezing point that is below the freezing point of either component diester. No monobenzoates of these glycols are disclosed or suggested in this patent as required or optional plasticizers.
The present invention provides liquid ester compositions comprising a mixture consisting essentially of:
at least one monoester represented by a formula selected from the group consisting of H[OR1]nOC(O)R2 and HOR3OC(O)R2; and
at least one diester represented by the formula R2(O)COCH2CH2OCH2CH2OC(O)R2;
wherein said diester is a solid at 28xc2x0 C., each R1 is alkylene containing 2 or 3 carbon atoms; R2 is phenyl or substituted phenyl, R3 is alkylene containing from 2 to 8 carbon atoms, n is 2 or 3, and wherein the concentration of said monoester in said ester compositions is sufficient to render said mixture liquid at 28xc2x0 C.
The concentration of monobenzoate required to yield a liquid composition is typically at least about 30 weight percent of the mono-/dibenzoate mixture. The present compositions can contain up to 99 weight percent of the present monobenzoates.
This invention also provides plasticized aqueous polymer compositions wherein the polymer is selected from the group consisting of 1) polymerized vinyl esters including but not limited to polyvinyl acetate, and copolymers of said esters with at least one olefin such as ethylene and 2) polymerized esters of ethylenically unsaturated carboxylic acids such as acrylic and methacrylic acids, including copolymers of these esters with olefins such as ethylene and styrene, and 3) condensation polymers such as polyesters and polyamides. The plasticizer comprises a liquid benzoic acid ester composition of the present invention.
The Dibenzoate Ingredient
The dibenzoate present in the liquid ester compositions of this invention is diethylene glycol (DEG) dibenzoate, represented by the formula R2(O)COCH2CH2OCH2CH2OC(O)R2; wherein R2 is phenyl or substituted phenyl.
Benzoic acids suitable for use in preparing the present ester compositions can be represented by the general formula HO(O)CAr, wherein Ar represents an unsubstituted or substituted phenyl radical. Substituents that can be present on the phenyl radical include but are not limited to alkyl radicals containing from 1 to about 4 carbon atoms and halogen atoms. Examples of substituted benzoic acids include but are not limited to ortho-, meta-, and para-toluic acid and the various halogenated benzoic acids such as o-chlorobenzoic acid.
The Monobenzoate Ingredient
In addition to the aforementioned dibenzoate, the liquid ester compositions of this invention include at least one monobenzoate derived from
a) a dialkylene glycol containing from 4 to 6 carbon atoms or a diol containing from 2 to 8 carbon atoms, and
b) benzoic acid or a substituted benzoic acid.
Preferred dihydric alcohols for preparing the monobenzoate are diethylene glycol, dipropylene glycol and dihydric alcohols containing from 3 to 6 carbon atoms. It is known that the two propylene radicals present in dipropylene glycol can exist in various isomeric forms, including xe2x80x94CH2CH2CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94CH(CH3)CH2xe2x80x94, and xe2x80x94CH2CH(CH3)xe2x80x94. The last two structural isomers have stereoisomeric forms.
In preferred ester compositions of this invention the hydrocarbon radicals represented by R1 are ethylene or a mixture of ethylene and at least one of the isomeric propylene radicals.
The diester ingredient of the present compositions can be prepared by reacting benzoic acid, a substituted benzoic acid or the corresponding acid halide with diethylene glycol (DEG). Alternatively, other methods such as transesterification can be used to prepare the diester.
Depending upon the desired molar ratio of mono- to dibenzoate in the final product, the molar ratio of the acid or acid halide to the glycol in the initial reaction mixture is typically from 1:1 to about 1.7:1, and the reaction is continued until substantially all of the initial acid has reacted.
The preference for unsubstituted benzoic acid as a reactant for preparing both the dibenzoate and monobenzoate esters of the present compositions is based on the cost and availability of this compound.
Minimum Monobenzoate Concentrations for a Liquid Composition
While diethylene glycol dibenzoate is an effective plasticizer for organic polymers, a disadvantage of this compound as a plasticizer for polymer compositions processed at ambient temperature is that it melts at 28xc2x0 C. The corresponding monobenzoates of DEG and DPG melt below 25xc2x0 C., however the prior art, including the aforementioned patent to Arendt, suggests that these monobenzoates are not as useful plasticizers as the dibenzoates because they were considered less compatible than the corresponding dibenzoates with aqueous emulsions containing acrylic and/or vinyl ester polymers.
The present invention is based on the discovery that mixtures containing the dibenzoate of DEG together with at least one monobenzoate of a dihydric alcohol containing from 2 to 8 carbon atoms, DEG and/or DPG are not only liquids at ambient temperature, but, in many instances, are equally effective plasticizers for many aqueous polymer emulsions as DEG dibenzoate used alone. For this reason the preferred monobenzoate(s) can constitute up to 99 percent of the present ester compositions.
The ability of DEG dibenzoate to form liquid blends with the corresponding monobenzoate and/or the monobenzoates of other dihydric alcohols and glycols, the concentration of monobenzoate required to achieve these blends and the efficacy of these blends as plasticizers for aqueous polymer compositions are not suggested in the prior art relating to this type of plasticizer.
When the monobenzoate is diethylene glycol (DEG) monobenzoate, a minimum monobenzoate concentration of about 30 weight percent in a blend with DEG dibenzoate is required to obtain a blend that is a liquid at 12xc2x0 C. Pure DEG monobenzoate exhibits a hydroxyl number of 264 and a glass transition temperature of xe2x88x9276xc2x0 C.
A convenient method for determining the relative concentration of monobenzoates in a mixture blend containing DEG dibenzoate is by measuring the hydroxyl number of the mixture.
The procedure for determining the hydroxyl number of a compound or a mixture is described in ASTM test procedure E222, method B.
Pure DEG dibenzoate contains no unreacted hydroxyl groups, and therefore exhibits a hydroxyl number of 0. Knowing the hydroxyl number for a particular monobenzoate/dibenzoate blend, the relative concentration of monobenzoate in the blend can readily be calculated or obtained from a curve of hydroxyl number vs. weight percent monobenzoate for the particular ester pair.
The hydroxyl numbers for the DEG monobenzoate/dibenzoate blends exemplified in the aforementioned patent to Arendt range from 12.1 to 58.4. This is equivalent to a DEG monobenzoate concentration in the blend of from about 4 to about 18 percent by weight. It is not disclosed in this patent that the blend containing 58.4 weight percent of the monobenzoate is a liquid at 28xc2x0 C.
Optional Ester Ingredients
In addition to the monobenzoate/dibenzoate mixtures described hereinbefore, the present ester compositions can include up to 70 weight percent of dipropylene glycol dibenzoate to enhance the desirable properties of the ester compositions, particularly lowering their freezing point.
Aqueous Polymer Compositions Suitable for Use with the Present Plasticizers
The liquid ester compositions of this invention are suitable for use with aqueous compositions containing at least one emulsified polymer. Preferred classes of monomers used to prepare aqueous polymer emulsions suitable for use with the present liquid ester compositions as plasticizers include but are not limited to 1) vinyl esters such as vinyl acetate and copolymers of these esters with olefins such as ethylene, 2) esters derived from ethylenically unsaturated acids, such as acrylic and methacrylic acids, and mono- or polyhydric alcohols, 2) condensation polymers such as polyesters and polyamides, 3) polymers derived from ethylenically unsaturated acids or their esters and ethylenically unsaturated hydrocarbons such as ethylene and styrene.
These aqueous polymer compositions are useful in numerous end-use applications, including but not limited to adhesives, caulks, coatings and polishes for a variety of applications.
The polymer selected will be determined at least in part by the intended end use of the polymer emulsion. Polymers of vinyl esters such as vinyl acetate and copolymers of these esters with olefins such as ethylene or with esters of acrylic or methacrylic acid are preferred materials for adhesives.
The aforementioned patent to Arendt relating to the dibenzoates of DEG and DPG as plasticizers in combination with small amounts of the corresponding monobenzoate discloses aqueous emulsions of acrylic polymers useful as latex caulks.
The accompanying examples demonstrate the improved level of desirable properties such as increased open time (the maximum time interval following application of an adhesive layer to a surface during which the layer will bond to a surface placed in contact with the adhesive) that can be achieved with no increase in set time. Set time is defined as the minimum time period required for an adhesive to bond to a surface in contact with the adhesive.